The present invention relates to control systems for controlling the operation of a wheelchair while climbing or descending stairs.
A major challenge for wheelchair designers has been to design a wheelchair which can ascend and descend stairs, and yet not be unduly large, cumbersome or expensive. One design is shown in U.S. Pat. No. 4,674,584. The wheelchair travels on normal wheels during horizontal operation, and has ultrasonic sensors detecting the presence of a stairway or other incline. The sensor signals are used to activate and lower a pair of tracks, which are looped endless treads. In addition to lowering the tracks, a signal from the ultrasonic sensors is also used to determine if the incline is too steep for the wheelchair to negotiate. In such an instance, the wheelchair will not be allowed to move forward and up or down the stairs.
One problem with movement down a stairway is that as a wheelchair edges over the stairway, it will suddenly tilt downward and slam onto the stairway, jolting the user or potentially injuring the user. A solution to this problem is described in U.S. Pat. No. 4,671,369. Forward and rearward arms are deployed beneath the wheelchair and extend downward over the stairs as the wheelchair approaches. As the body of the wheelchair begins to tilt down the stairs, the arm is already resting across the steps. A shock absorbing, fluid-filled cylinder between this extended arm and the body of the wheelchair ensures that the body of the wheelchair will slowly ease into position pointing down the stairway. The shock absorber is simply a tube with a piston extending through it and fluid therein to slow the movement of the piston through the cylinder. The '369 patent shows a mechanical linkage mechanism for deploying these cushioning arms.
In order to provide maximum comfort for a user during the ascending or descending of stairs, the seat is tilted so that the user is held horizontal while the body of the wheelchair is inclined. This tilting movement is also necessary to move the center of gravity of the wheelchair and the user to an appropriate position to allow it to safely climb the stairs. If the center of gravity is too far forward, away from the stairs, the wheelchair might roll. Thus, there is a danger, that if the tilting mechanism fails to operate, the wheelchair could roll.